The present invention relates to an air-fuel ratio control system for an internal combustion engine for a motor vehicle provided with a three-way catalytic converter in an exhaust system, and with an O.sub.2 -sensor for detecting the oxygen concentration of exhaust gases, and more particularly to a feedback control system for controlling air-fuel ratio during rapid acceleration and deceleration of the motor vehicle.
The air-fuel ratio control system responds to the feedback signal from the O.sub.2 -sensor to control the air-fuel ratio of the air-fuel mixture to a stoichiometric air-fuel ratio at which ratio the three-way converter is most effective. Such a feedback control operates under conditions of a coolant temperature higher than a predetermined value and when the activated O.sub.2 -sensor has a higher body temperature of the sensor than a predetermined value. When either the coolant temperature or O.sub.2 -sensor temperature is lower than the predetermined values, the feedback control system is disabled and the air-fuel ratio control signal is fixed to a constant value to hold the air-fuel ratio at a fixed value.
On the other hand, even if the feedback control operates under the normal conditions, when an accelerator pedal of the motor vehicle is fully depressed, the air-fuel ratio is fixed to enrich the mixture so as to meet the acceleration of the vehicle. Japanese Patent Laid Open No. 53-13021 discloses an example of such an enrichment system.
In order to increase the emission control range in engine operation, it is preferable to set the predetermined temperature, at which the feedback control system is enabled, to a lower value. On the contrary, in order to insure driveability during warming-up of the engine, it is preferable to set the temperature, at which the air-fuel ratio is fixed to enrich the mixture, to a higher value. To meet both requirements, usually the coolant temperature as the condition for enabling the feedback control system is set to a value between 20.degree. and 50.degree. C.
However, intake air is not sufficiently pre-heated in the intake manifold of the engine at a coolant temperature below 80.degree. C., so that unvaporized fuel sticks to the inner wall of the intake manifold. The sticking of fuel to the wall causes the response of the feedback control operation to be delayed, increasing the amplitude of the oscillation of the system which increases deviation of the air-fuel ratio from the stoichiometric air-fuel ratio. Such a large deviation of the air-fuel ratio is further increased during rapid acceleration and rapid deceleration, causing an excessively rich or lean air-fuel mixture which causes the emission control effect to deteriorate. Japanese Patent Laid Open No. 56-162250 discloses an air-fuel ratio control system which operates to enrich the air-fuel mixture to a maximum rich value at a cold engine when rapidly accelerated. However, it is not always desirable to provide the maximum rich mixture.